|Migration across the North Sea studied by radar: 1. Survey through the year|In: Ibis. British Ornithologists' Union/Wiley: London. ISSN 0019-1019, more
1 Migration across the North Sea off Norfolk is reviewed over three years on the basis of the detection of bird echoes by radar. 2 Large movements are most frequent in March, April and November and least frequent in December and January. 3 Most movements are east between January and May, west in June and July, and west or southwest during the rest of the year, while there is also a small passage northwest in spring and southeast in autumn. 4 Occasional movements were observed southwest in March in cold weather, west in spring during eastward emigrations with fog or rain at sea, also a presumed drift-migration northwest in March and possibly southwest in September, with subsequent redetermined movements heading southeast and east respectively. 5 The chief additions to existing knowledge based on visual evidence are (a) the huge eastward emigrations in spring, (b) the arrivals southwest, presumably direct from Scandinavia, in late autumn and (c) the presumed redetermined movements eastward after drifted arrivals. 6 The limicoline species in June and July, and the passerine species in late autumn, cross the North Sea primarily with easterly winds and hardly at all with westerly winds, whereas in spring they often set out against the wind. This might merely be due to a stronger migratory urge in spring than autumn, but it also seems possible that westerly winds deter migration in autumn because they are so often associated with rain and other unfavourable conditions.